A multi-model evaluation of aerosols over South Asia: common problems and possible causes

Atmos. Chem. Phys., 15, 5903–5928, 2015 www.atmos-chem-phys.net/15/5903/2015/ doi:10.5194/acp-15-5903-2015 © Author(s) 2015. CC Attribution 3.0 License. A multi-model evaluation of aerosols over South Asia: common problems and possible causes X. Pan1 , M. Chin1 , R. Gautam2 , H. Bian1,3 , D. Kim1,4 , P. R. Colarco1 , T. L. Diehl1,4,* , T. Takemura5 , L. Pozzoli6 , K. Tsigaridis7,8 , S. Bauer7,8 , and N. Bellouin9 1 NASA Goddard Space Flight Center, Greenbelt, MD, USA 2 Centre of Studies in Resources Engineering and Interdisciplinary Program in Climate Studies, Indian Institute of Technology, Bombay, Mumbai, India 3 Joint Center for Earth Systems Technology, University of Maryland Baltimore City, Baltimore, MD, USA 4 Universities Space Research Association, Columbia, MD, USA 5 Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan 6 Eurasia Institute of Earth Sciences, Istanbul Technical University, Istanbul, Turkey 7 NASA, Goddard Institute for Space Studies, New York, NY, USA 8 Center for Climate Systems Research, Columbia University, New York, NY, USA 9 Department of Meteorology, University of Reading, Reading, Berkshire, United Kingdom * current address: European Commission at the Joint Research Center, Ispra, Italy Correspondence to: X. Pan () Received: 6 June 2014 – Published in Atmos. Chem. Phys. Discuss.: 22 July 2014 Revised: 15 April 2015 – Accepted: 3 May 2015 – Published: 28 May 2015 Abstract. Atmospheric pollution over South Asia attracts special attention due to its effects on regional climate, water cycle and human health. These effects are potentially growing owing to rising trends of anthropogenic aerosol emissions. In this study, the spatio-temporal aerosol distributions over South Asia from seven global aerosol models are evaluated against aerosol retrievals from NASA satellite sensors and ground-based measurements for the period of 2000–2007. Overall, substantial underestimations of aerosol loading over South Asia are found systematically in most model simulations. Averaged over the entire South Asia, the annual mean aerosol optical depth (AOD) is underestimated by a range 15 to 44 % across models compared to MISR (Multi-angle Imaging SpectroRadiometer), which is the lowest bound among various satellite AOD retrievals (from MISR, SeaWiFS (Sea-Viewing Wide Field-of-View Sensor), MODIS (Moderate Resolution Imaging Spectroradiometer) Aqua and Terra). In particular during the postmonsoon and wintertime periods (i.e., October–January), when agricultural waste burning and anthropogenic emissions dominate, models fail to capture AOD and aerosol absorption optical depth (AAOD) over the Indo–Gangetic Plain (IGP) compared to ground-based Aerosol Robotic Network (AERONET) sunphotometer measurements. The underestimations of aerosol loading in models generally occur in the lower troposphere (below 2 km) based on the comparisons of aerosol extinction profiles calculated by the models with those from Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) data. Furthermore, surface concentrations of all aerosol components (sulfate, nitrate, organic aerosol (OA) and black carbon (BC)) from the models are found much lower than in situ measurements in winter. Several possible causes for these common problems of underestimating aerosols in models during the post-monsoon and wintertime periods are identified: the aerosol hygroscopic growth and formation of secondary inorganic aerosol are suppressed in the models because relative humidity (RH) is biased far too low in the boundary layer and thus foggy conditions are poorly represented in current models, the nitrate aerosol is either missing or inadequately accounted for, and emissions from agricultural waste burning and biofuel usage are too low in the emission inventories. These common problems and possible causes found in multiple models point out directions for future model improvements in this important region. Published by Copernicus Publications on behalf of the European Geosciences Union. 5904 1 X. Pan et al.: A multi-model evaluation of aerosols over South Asia Introduction South Asia, particularly the Indo–Gangetic Plain (IGP) bounded by the towering Himalaya (Fig. 1), is one of the global hotspots with persistent high aerosol optical depth (AOD) routinely observed by satellite remote sensors (e.g., Moderate Resolution Imaging Spectroradiometer – MODIS, Multi-angle Imaging SpectroRadiometer – MISR and SeaViewing Wide Field-of-View Sensor – SeaWiFS), as well as from ground-based measurements (e.g., Aerosol Robotic Network – AERONET). The potential influence of aerosols on the climate and water cycle in this region (e.g., Indian summer monsoon) via surface dimming and atmospheric warming has been widely discussed in the literature (e.g., Ramanathan et al., 2005; Lau et al., 2006). The atmospheric heating due to absorbing aerosols (mainly from black carbon – BC) is estimated to be large especially in the wintertime, about 50–70 W m−2 (Ganguly et al., 2006). Recent studies have shown that the depositions of absorbing aerosols such as BC and dust over Himalaya are linked to snow albedo reduction and accelerated snow/ice melt in Himalaya during the pre-monsoon season (Lau et al., 2010; Qian et al., 2011; Yasunari et al., 2010; Gautam et al., 2013). Besides these climate impacts, fine aerosol particles (PM2.5 ) are known to affect public health, especially over IGP where large portions of the Indian population live. At Delhi, for example, PM2.5 concentration in 2007 was 97 ± 56 µg m−3 (Tiwari et al., 2009), 9 times the air quality guidelines recommended by the World Health Organization in 2005. Increases in anthropogenic aerosol emissions and loading in South Asia in recent decades have been well documented (Ohara et al., 2007; Hsu et al., 2012; Kaskaoutis et al., 2012; Babu et al., 2013), in contrast with the decreasing emission trends over Europe and North America (Granier et al., 2011; Diehl et al., 2012). Therefore, it is critical to accurately represent aerosol sources, distributions and properties in models over this heavily polluted region in order to project the future climate and air quality changes in South Asia with confidence. Previous studies, however, reported that global models generally underestimated aerosol loading over South Asia, especially over the IGP in winter (Reddy et al., 2004; Chin et al., 2009; Ganguly et al., 2009; Henriksson et al., 2011; Goto et al., 2011; Cherian et al., 2013; Sanap et al., 2014). Among them, Ganguly et al. (2009) reported that the GFDL-AM2 model largely underestimated the AOD over the IGP during winter by a factor of 6. Recently, AOD simulated by the regional climate model (RegCM4) showed higher correlation with AERONET AOD at stations over dust-dominated areas in south Asia than over the regions dominated by anthropogenic aerosols, i.e., 0.71 vs. 0.47 (Nair et al., 2012). Eleven out of twelve models participating in the Aerosol Compari (...truncated)